Reducing levels of cat fur and dust mites is unlikely to prevent asthma in children, a study has shown.

For the last two decades, researchers said the risk of developing an allergy is linked with early exposure to dust.

However, the study, which examined children in Ashford, Kent, challenged this theory.

Instead, it found children were more likely to suffer from conditions if their father was susceptible or if the child was a firstborn.

Living room dust

Researchers monitored 625 youngsters from birth to the age of five-and-a-half, and eventually tested 552 children for sensitivity to house dust mites, cat fur and grass pollens.

The mothers were interviewed every year to find out if their children had suffered wheezing in the past 12 months.

The more research you do, the more complex you realise it is

Dr Warren Lenney

Scientists from Imperial College School of Medicine at the National Heart and Lung Institute in London also collected house dust samples from the families' living room floors when the youngsters were eight weeks old.

The study, published in the medical journal Thorax, said there were no significant links between levels of exposure to allergens measured in dust samples and sensitivity or frequency of wheezing.

Long-term effect

Dr Warren Lenney, of the British Thoracic Society, said the report highlighted the complexity of the issue.

The consultant respiratory paediatrician, who works at the University Hospital of North Staffordshire, said there was no doubt there was a relationship between the environment and asthmatic symptoms.

But the study "confirmed the fact that there is a very complex relationship in what is built into us genetically and what is in the environment".

Dr Lenney told BBC News Online: "The more research you do, the more complex you realise it is."

He said out of all the environmental studies undertaken into the causes of asthma, the only one to have shown any real long-term effect was linked to farms.

"Children brought up on farms, irrespective of which country, have a reduced prevalence of developing asthma and other atopic conditions," he said.